The present invention relates to a valve timing control device for controlling the timing of opening of intake and exhaust valves.
As this type of valve timing control device, a valve timing control device for an internal combustion engine disclosed in Japanese Patent Unexamined Publication No. 63-131808 has been conventionally known.
This device includes a gear movably provided between a timing pulley and a camshaft in such a manner that the timing pulley and the camshaft are rotated relative to each other by moving the gear therebetween, so as to vary the timing of opening and closing of valves. Further, hydraulic chambers are formed on the front and rear sides of the gear, and hydraulic pressure supply means are provided for supplying hydraulic pressure to these two hydraulic chambers through a cam journal portion.
By controlling the hydraulic pressure supplied to the two hydraulic chambers by the above-mentioned hydraulic pressure supply means, the gear is moved in a desired direction or stopped/retained at a desired position between the cam pulley and the camshaft. Thus, the value timing is controlled to a desired timing in accordance with an operating condition.
In the above-described conventional technique, rotation of the timing pulley is transmitted to the camshaft through helical splines formed on the inner and outer peripheries of the gear. Consequently, reaction force of the driving torque of the camshaft is constantly applied to the gear, and the camshaft always tends to be delayed from rotation of the timing pulley due to the frictional force. Therefore, owing to the reaction force from the camshaft, the gear is applied with a force which moves the gear in a direction to vary the valve timing to a delaying side.
In order to control the value timing to a desired timing, the gear is retained at a desired position by controlling the hydraulic pressure supplied to the two hydraulic chambers. At this time, due to the above-mentioned force applied to the gear so as to vary the valve timing to the delaying side, the hydraulic pressure in the hydraulic chamber which receives this force (i.e., the advancing-side hydraulic chamber) is increased. Such an increase in the hydraulic pressure causes fluid to leak to the outside from the advancing-side hydraulic chamber, thereby decreasing an amount of fluid in the hydraulic chamber. Since the gear is moved due to such a decrease in the fluid amount, it is feared that the gear can not be retained at the desired position.
Although such a problem may arise when retaining the gear at the desired position, measures against this problem were not taken into consideration in the conventional technique described above.